Iodoacetate ribonuclease

The presence of homoserine, homoserine lactone, and acid hydrolysis of ribonuclease, 50 % inactivated by iodoacetate at pH 2.8, and the poor recovery of methionine provides another example for this type of breakdown of methionine sul-fonium salts (Gundlach et al, 1959b). The same products in varying proportions were observed in the decomposition of methionine carboxymethyl-... 

The characterization of proteins and their derivatives is outside the scope of this monograph. It is necessary to emphasize, however, that modification of native proteins frequently gives rise to complex mixtures of products. The complexity of the situation is not inunediately apparent solely from the stoichiometry of the modification reaction. The now classical case of the reaction of native bovine pancreatic ribonuclease A with iodoacetic acid at pH 5.5 may be cited in this context. One carboxymethyl group is introduced per molecule of... 

The imidazole ring of histidine also undergoes alkylation on reaction with iodoacetate. In ribonuclease, two residues (His 12 and His 119) are alkylated with loss of activity when the enzyme is treated with iodoacetate at pH 5.5. 

Ribonuclease A contains several disulfide bonds but no free sulfhydryl group. The molecular mass of the protein measured before and after reduction and alkylation (with iodoacetic acid) was found to be 13,682 and 14,155 Da, respectively. How many disulfide bonds does this protein contain ... 

Group-specific reagents there are a variety of reagents which can specifically modify functional groups found in enzymes, e.g. iodoacetate at pH 5.5 was employed to identify specific histidine residues important in the catalytic activity of pancreatic ribonuclease. The other histidine residues in this enzyme molecule are less reactive with iodoacetate. 

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